An electric power system which can supply electric power to a load using a secondary battery and charge the secondary battery, if needed, even when the load is in operation, is widely known (for instance, a hybrid car, an electric vehicle, etc.). In this kind of electric power system, in order to suppress the performance degradation of the above-mentioned secondary battery or to increase the efficiency of operation of the electric power system, various devices which estimate the charge status (SOC [%]: SOC is the abbreviation for State of Charge) of the above-mentioned secondary battery have been proposed conventionally.
For example, SOC can be estimated by integrating detection values of a current under charge and discharge of the above-mentioned secondary battery. However, by this technique, estimate of SOC may deviate from a true value since errors of current detection values are accumulated in the estimate of SOC.
Then, techniques for accurately estimating SOC by correcting SOC estimate obtained by integrating current detection values using SOC estimate otherwise computed based on an open circuit voltage, or carrying out weighted combination of both of them, have been proposed conventionally (for example, refer to Japanese Patent Application Laid-Open (kokai) No. H11-223665, Japanese Patent Application Laid-Open (kokai) No. 2000-150003, Japanese Patent Application Laid-Open (kokai) No. 2005-201743, Japanese Patent Application Laid-Open (kokai) No. 2007-192726 and Japanese Patent Application Laid-Open (kokai) No. 2008-243373, etc.).